using System;
using System.Text.RegularExpressions;
public class Example
{
public static void Main()
{
string pattern = @"^ # beginning of string
(?:
. # match a single character
(?=
.*?
# if we already have captured the end of the string
(?(1)
# make sure we do not go past the correct position
(?= .\1$ )
)
# capture the end of the string +1 character, adding to \1 every iteration
( .\1?$ )
)
)* # repeat
# the middle character follows, capture it
(.)";
string input = @"1
123
12345
1234567
123456789
12345678901
1234567890123
123456789012345
";
RegexOptions options = RegexOptions.IgnorePatternWhitespace | RegexOptions.Multiline;
foreach (Match m in Regex.Matches(input, pattern, options))
{
Console.WriteLine("'{0}' found at index {1}.", m.Value, m.Index);
}
}
}
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for C#, please visit: https://msdn.microsoft.com/en-us/library/system.text.regularexpressions.regex(v=vs.110).aspx